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Abstract:

An image forming apparatus includes: a rotatable belt member; a toner
image forming unit which forms a toner image on the belt member; a first
detection unit which detects the density of the toner image transferred
to the belt member; a plurality of position marks which detects a
position in the circumferential direction of the belt member; a second
detection unit which detects the position marks; a calculation unit which
calculates the position in the circumferential direction of the belt
member based on a detection result of the second detection unit; and an
adjustment unit which adjusts toner image forming conditions of the toner
image forming unit based on the output of the first detection unit and
the circumferential direction position calculated by the calculation
unit; wherein the shape of each of the plurality of position marks is
different at each circumferential direction position.

Claims:

1. An image forming apparatus comprising:a rotatable belt member;a toner
image forming unit which forms a toner image on the belt member;a first
detection unit which is arranged facing the belt member and detects the
toner image transferred to the belt member;a plurality of position marks
which is provided in the plural points on the belt member in the
circumferential direction of the belt member;a second detection unit
facing the belt member which detects the position marks;a calculation
unit which calculates the position in the circumferential direction of
the belt member based on a detection result of the second detection unit;
andan adjustment unit which adjusts toner image forming conditions of the
toner image forming unit based on the output of the first detection unit
and the circumferential direction position calculated by the calculation
unit;wherein the circumferential shape of each of the plurality of
position marks is different each other.

2. The image forming apparatus according to claim 1, wherein the shape of
each of the position marks is different in the dimension in the
circumferential direction of the belt member.

3. The image forming apparatus according to claim 1, wherein the shape of
each of the position marks is different in the dimension in the width
direction of the belt member.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to an image forming apparatus of a
copying machine or printer which uses an electrophotographic system or an
electrostatic recording system. More particularly, the present invention
is related to an image forming apparatus capable of recognizing the
density of a toner image transferred to an intermediate transfer belt
corresponding to a position in the circumferential direction of the
intermediate transfer belt.

[0003]2. Description of the Related Art

[0004]Conventionally, for example, as an image forming apparatus capable
of forming a full color image, the following direct transfer--and
intermediate transfer--type image forming apparatuses are known. In a
direct transfer-type image forming apparatus, a toner image formed on a
single or a plurality of photosensitive drums is transferred to a
transfer material carried on a belt member (hereinafter referred to as a
"transfer belt"), which is a transfer material bearing member capable of
rotating in the circumferential direction. In an intermediate
transfer-type image forming apparatus, a toner image formed on a single
or a plurality of photosensitive drums is transferred once (primary
transfer) to a belt member (hereinafter referred to as an "intermediate
transfer belt"), which is an intermediate transfer member capable of
rotating in the circumferential direction. Subsequently, in an
intermediate transfer-type image forming apparatus, the toner image on
the intermediate transfer belt is transferred (secondary transfer) to a
transfer material. In an intermediate transfer-type image forming
apparatus, it is easy to form an image on various recording materials,
and thus the selectivity of the recording material can be increased.

[0005]In an image forming apparatus, the density of a patch image is
detected by an optical sensor and the image density of the colors is
controlled by a control of toner replenishment of developer by a patch
detection system or the like, and registration deviation is detected by
the optical sensor and registration correction is performed.

[0006]Given the above, a constitution of an image forming apparatus which
has an intermediate transfer belt and controls the image density based on
detection of a patch density by an optical sensor can be envisioned. A
technology called ground correction is necessary for the measurement of
the patch density transferred to a transfer surface of an intermediate
transfer belt by an optical sensor. During measurement of the patch
density, ground correction has an effect of canceling the influence from
the gloss of the surface of the ground on the patch density, while taking
the condition of the ground of the intermediate transfer belt to which
the toner is transferred into account. The condition of the ground of the
intermediate transfer belt includes, for example, variation over time in
the gloss of the transfer surface of the intermediate transfer belt and
gloss irregularities in the intermediate transfer belt within one round.
In particular, as an invention for canceling the influence from gloss
irregularities in the intermediate transfer belt within one round on the
patch density, the image forming apparatus disclosed in Japanese Patent
Application Laid-Open No. 05-150574 has been proposed.

[0007]In the image forming apparatus disclosed in Japanese Patent
Application Laid-Open No. 05-150574, before measuring the patch density,
a control apparatus reads one round of the transfer surface of the
intermediate transfer belt which becomes the ground of the toner image
via a density sensor, and then stores the phase of the intermediate
transfer belt and the output value of the density sensor for each phase.
Subsequently, the control apparatus references the phase of the transfer
surface of the intermediate transfer belt and the output value of the
density sensor in each phase which were stored to ascertain the output
value of the density sensor corresponding to the transfer surface of the
intermediate transfer belt of a certain phase which becomes the ground of
the toner patch. The control apparatus then performs ground correction to
obtain the density of the toner patch.

[0008]Further, in the image forming apparatus disclosed in Japanese Patent
Application Laid-Open No. 05-150574, in order for the control apparatus
to become able to recognize the phase of the transfer surface of the
intermediate transfer belt, marks are attached to the intermediate
transfer belt at prescribed intervals in the circumferential direction,
and a mark detecting sensor which detects the marks is arranged facing
the intermediate transfer belt. A calculation unit within the control
apparatus determines the current phase of the intermediate transfer belt
based on the elapsed time from when the prescribed marks on the
intermediate transfer belt are detected by the mark position detecting
sensor.

[0009]However, when obtaining a profile of one round of the intermediate
transfer belt using an optical sensor, a reference position which becomes
the reference of the circumferential direction is necessary for the
intermediate transfer belt. If there is one mark attached to such a
reference position of the intermediate transfer belt, a wait time until
the mark comes to the location of the mark detecting sensor occurs.
Therefore, in the image forming apparatus disclosed in Japanese Patent
Application Laid-Open No. 05-150574, in order to reduce the wait time
until the mark comes to the location of the mark detecting sensor, a
plurality of marks of the same shape and same pattern are attached to the
intermediate transfer belt.

[0010]However, the belt position detecting marks cannot be distinguished
from each other, and after a power source is suddenly turned OFF or after
a process to eliminate a paper jam, the reference position is lost, and
the reference position and the profile of one round of the intermediate
transfer belt must be retaken, and thus a wait time may occur.

[0011]The present invention provides an image forming apparatus which can
reduce the time for determining the position of a belt member.

SUMMARY OF THE INVENTION

[0012]An image forming apparatus of the present invention includes a
rotatable belt member, a toner image forming unit which forms a toner
image on the belt member, a first detection unit which is arranged facing
the belt member and detects the toner image transferred to the belt
member, a plurality of position marks which is provided in the plural
points on the belt member in the circumferential direction of the belt
member, a second detection unit facing the belt member which detects the
position marks, a calculation unit which calculates the position in the
circumferential direction of the belt member based on a detection result
of the second detection unit, and an adjustment unit which adjusts toner
image forming conditions of the toner image forming unit based on the
output of the first detection unit and the circumferential direction
position by the calculation unit, wherein the circumferential shape of
each of the plurality of position marks is different each other.

[0013]Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross-section view illustrating the constitution of an
image forming apparatus according to a first embodiment of the present
invention;

[0015]FIG. 2 is an enlarged cross-section view of FIG. 1 illustrating the
constitution of an image forming portion;

[0016]FIGS. 3A and 3B are cross-section views illustrating the
constitution of an optical sensor;

[0017]FIGS. 4A to 4C are schematic views illustrating the shapes of blank
seals extending in the circumferential direction of an intermediate
transfer belt;

[0018]FIGS. 5A and 5B are schematic views illustrating the shapes of blank
seals of an intermediate transfer belt according to a second embodiment;
and

[0019]FIGS. 6A and 6B are schematic views illustrating the shapes of blank
seals of an intermediate transfer belt according to a third embodiment.

DESCRIPTION OF THE EMBODIMENTS

[0020]Exemplary embodiments of the present invention will be described
below in detail with reference to the drawings. However, the dimensions,
materials, and shapes of the constitutional members and the relative
positions thereof described in each embodiment below can be appropriately
modified depending on the constitution of the apparatus to which the
present invention is applied and various other conditions. Therefore,
unless specific descriptions are particularly mentioned, the scope of the
present invention is not limited to the embodiments described below.

First Embodiment

[0021]FIG. 1 is a cross-section view illustrating the constitution of an
image forming apparatus 100 according to a first embodiment of the
present invention. The image forming apparatus 100 is a full color
electrophotographic image forming apparatus which has four photosensitive
drums 1a to 1d, uses an intermediate transfer system, and utilizes an
electrophotographic image forming process. As illustrated in FIG. 1, the
image forming apparatus 100 has an image forming apparatus main body
(hereinafter referred to as "apparatus main body") 100A. The image
forming apparatus 100 has "a plurality of", i.e. four, "image forming
portions", which are a first image forming portion Sa, a second image
forming portion Sb, a third image forming portion Sc, and a fourth image
forming portion Sd, within the apparatus main body 100A. Each image
forming portion Sa, Sb, Sc, Sd is for forming each color of yellow,
magenta, cyan, and black, respectively. In the first embodiment, the
constitutions of the image forming portions Sa to Sd are substantially
the same except that the color of toner used is different. Accordingly,
in the following description, if it is not particularly necessary to
distinguish them, the additional letters a, b, c, and d added to the
reference numerals will be omitted and the term of the image forming
portions S will be generally used in order to show that these are an
element provided for one of the colors.

[0022]The image forming portions S are equipped with photosensitive drums
1a to 1d, which are a plurality of "image bearing members" on whose
surface toner images are formed. On the periphery of the photosensitive
drums 1, charging rollers 2a to 2d as first charging portions and laser
scanners 3a to 3d as exposing portions are arranged sequentially along
the rotation direction of the photosensitive drums 1. Further, on the
periphery of the photosensitive drums 1, developing apparatuses 4a to 4d
as developing units and drum cleaners 6a to 6d as drum cleaning units are
arranged sequentially along the rotation direction of the photosensitive
drums 1. An intermediate transfer belt 51, which is an endless "belt
member" having a transfer surface 51a (refer to FIG. 2) onto which the
toner images of the plurality of photosensitive drums 1a to 1d are
transferred is arranged facing (adjacent to) the plurality of
photosensitive drums 1a to 1d. The image forming portions have a function
as a toner image forming unit which forms a toner image on the
intermediate transfer belt 51.

[0023]The intermediate transfer belt 51 is stretched over a drive roller
52, a tensing roller 55, a secondary transfer inner roller 56, and an
upstream control roller 65, which act as a plurality of support members.
A driving force is transmitted to the intermediate transfer belt 51 by
the drive roller 52, which is a belt driving unit, to move the belt 51 in
a rotating fashion in the direction of arrow R3. Primary transfer rollers
53a to 53d are arranged as primary transfer members at positions opposite
to the photosensitive drums 1a to 1d on the inner circumferential side of
the intermediate transfer belt 51. The intermediate transfer belt 51 is
biased toward the photosensitive drums 1a to 1d by the primary transfer
rollers 53a to 53d, and primary transfer portions (primary transfer nips)
N1a to N1d, at which the photosensitive drums 1a to 1d contact the
intermediate transfer belt 51, are formed. Further, a secondary transfer
outer roller 57 is arranged as a secondary transfer member at a position
opposite to the secondary transfer inner roller 56 on the outer
circumferential side of the intermediate transfer belt 51. The secondary
transfer outer roller 57 contacts the outer circumferential surface of
the intermediate transfer belt 51 to form a secondary transfer portion
(secondary transfer nip) N2. Images on the photosensitive drums 1a to 1d
formed by the image forming portions Sa to Sd are successively
multi-layer transferred (primary transfer step) onto the intermediate
transfer belt 51 contacting and passing over the photosensitive drums 1a
to 1d. Subsequently, the images transferred onto the intermediate
transfer belt 51 are further transferred (secondary transfer step) to a
recording material P, such as paper, at the secondary transfer portion
N2.

[0024]The image forming apparatus 100 described above operates as
described below. First, in the case of forming a full color image, the
photosensitive drums 1a to 1d are uniformly charged by the charging
rollers 2a to 2d, and then light exposure in accordance with an image
signal is carried out by the laser scanners 3a to 3d. Thereby,
electrostatic images are formed on the photosensitive drums 1a to 1d.
Subsequently, toner images are developed by the developing apparatuses 4a
to 4d, and a transfer bias is applied from a transfer high-voltage power
source (not illustrated) to the primary transfer rollers 53a to 53d to
successively transfer the toner images on the photosensitive drums 1a to
1d to the intermediate transfer belt 51. At this time, the intermediate
transfer belt 51 is arranged to be in contact with the photosensitive
drums 1a to 1d of the four colors due to the placement of the upstream
control roller 65 which control the position of the intermediate transfer
belt 51. The transfer residual toner remaining on the photosensitive
drums 1a to 1d is collected by the drum cleaners 6a to 6d. The images
successively multi-layer transferred onto the intermediate transfer belt
51 from each photosensitive drum 1a to 1d by the above-described
procedures are transferred to the recording material P by the application
of a secondary transfer bias between the secondary transfer inner roller
56 and the secondary transfer outer roller 57, which are the secondary
transfer members. The toner images on the recording material P are fixed
by a fixing apparatus 7, and thereby a full color image is obtained.

[0025]In the image forming apparatus 100, the density of a patch image is
detected by an optical sensor 30, and the image density of the colors is
controlled by a control of toner replenishment of developer by a patch
detection system, and registration deviation is detected by the optical
sensor 30 and registration correction is performed.

[0026]FIG. 2 is an expanded cross-section view of FIG. 1 illustrating the
constitution of the image forming portion S. The photosensitive drum 1 is
rotatably supported by the apparatus main body 100A (refer to FIG. 1). As
illustrated in FIG. 2, the photosensitive drum 1 is a cylindrical
electrophotographic photosensitive member basically including an aluminum
conductive substrate 11 and a photoconductive layer 12 formed on the
outer circumference of the substrate 11. The photosensitive drum 1 has a
shaft 13 at its center. The photosensitive drum 1 is rotatably driven in
the direction of arrow R1 centered on the shaft 13 by a driving unit (not
illustrated). In the first embodiment, the charging polarity of the
photosensitive drum 1 is negative.

[0027]A charging roller 2 is arranged above the photosensitive drum 1 as a
primary charging portion. The charging roller 2 abuts the surface of the
photosensitive drum 1, and uniformly charges the surface of the
photosensitive drum 1 with a prescribed polarity and potential. The
charging roller 2 has a conductive metal core 21 positioned in the
center, a low resistivity conductive layer 22 formed on the outer
circumference of the core 21, and a mid resistivity conductive layer 23,
and the charging roller 2 is formed overall in a roller shape. In the
charging roller 2, both ends of the metal core 21 are rotatably supported
by shaft bearing members (not illustrated), and arranged parallel to the
photosensitive drum 1. The shaft bearing members of both ends are biased
toward the photosensitive drum 1 by a compression unit (not illustrated).
Thereby, the charging roller 2 is pressed to the surface of the
photosensitive drum 1 with a prescribed compressive force. The charging
roller 2 dependently rotates in the direction of arrow R2 in accordance
with the rotation in the direction of arrow R1 of the photosensitive drum
1. A charging bias voltage is applied to the charging roller 2 by a
charging bias power source 24, which is a charging bias output unit.
Thereby, the surface of the photosensitive drum 1 is uniformly contact
charged.

[0028]A laser scanner 3 is arranged on the downstream side of the charging
roller 2 in the rotation direction of the photosensitive drum 1. The
laser scanner 3 scans while turning a laser beam ON and OFF based on
image information, and exposes light on the photosensitive drum 1.
Thereby, an electrostatic image (latent image) according to the image
information is formed on the photosensitive drum 1.

[0029]A developing apparatus 4 is arranged on the downstream side of the
laser scanner 3 in the rotation direction of the photosensitive drum 1.
The developing apparatus 4 has a developing container 41 which contains a
two-component developer including non-magnetic toner particles (toner)
and magnetic carrier particles (carrier) as developer. A developing
sleeve 42 as a developer bearing member is rotatably arranged within an
opening of the developing container 41 facing the photosensitive drum 1.
A magnet roller 43 as a magnetic field source unit is fixedly arranged
within the developing sleeve 42 so that it cannot rotate relative to the
rotation of the developing sleeve 42. The two-component developer is
carried on the developing sleeve 42 by the magnetic field formed by the
magnet roller 43. A control blade 44, which is a developer control member
for controlling and thinning the two-component developer carried on the
developing sleeve 42, is arranged under the developing sleeve 42. The
inside of the developing container 41 is divided into a developing
chamber 45 and a stirring chamber 46, and a replenishing chamber 47 which
contains toner for replenishment is provided above the two chambers 46
and 47. The replenishing chamber 47 replenishes toner to the stirring
chamber 46 during image density control by a patch detection system, and
is used for toner replenishment control which maintains the toner density
of the developer within the developing container 41 at a prescribed
value.

[0030]A thin layer of the two-component developer on the developing sleeve
42 is conveyed to a developing region opposite to the photosensitive drum
1 in accordance with the rotation of the developing sleeve 42. The
two-component developer on the developing sleeve 42 rises up in brush
shape in the developing region by the magnetic force of the developing
main pole of the magnet roller 43 positioned in the developing region,
and a magnetic brush of the two-component developer is formed. The
surface of the photosensitive drum 1 is abraded by the magnetic brush,
and a developing bias voltage is applied to the developing sleeve 42 by a
developing bias power source 48, which is a developing bias output unit.
Thereby, toner attached to a carrier which constitutes the head of the
magnetic brush attaches to the exposure portion of the electrostatic
image on the photosensitive drum 1 to form a toner image. In the first
embodiment, a toner image is formed on the photosensitive drum 1 by
reversal development in which toner which is charged to the same polarity
as the charging polarity of the photosensitive drum 1 is attached to
portions on the photosensitive drum 1 at which the electric charge is
attenuated by light exposure.

[0031]A primary transfer roller 53 is arranged below the photosensitive
drum 1 on the downstream side of the developing apparatus 4 in the
rotation direction of the photosensitive drum 1. The primary transfer
roller 53 includes a metal core 531 and a conductive layer 532 formed in
a cylindrical shape on the outer circumference of the core 531. Both ends
of the primary transfer roller 53 are biased toward the photosensitive
drum 1 by a compression member (not illustrated) such as a spring.
Thereby, the conductive layer 532 of the primary transfer roller 53 is
pressed to the surface of the photosensitive drum 1 via the intermediate
transfer belt 51 with a prescribed compressive force. A primary transfer
bias power source 54 is connected as a primary transfer bias output unit
to the metal core 531. A primary transfer portion N1 (N1a, N1b, N1c, N1d)
is formed between the photosensitive drum 1 and the primary transfer
roller 53. The intermediate transfer belt 51 is sandwiched at the primary
transfer portion N1. The intermediate transfer belt 51 moves in the
direction of arrow K1. The primary transfer roller 53 contacts the inner
circumferential surface of the intermediate transfer belt 51 and rotates
in accordance with the movement of the intermediate transfer belt 51.
During image formation, a primary transfer bias voltage, which is of an
opposite polarity (second polarity: positive polarity in the first
embodiment) relative to the normal charging polarity (first polarity:
negative polarity in the first embodiment) of the toner, is applied to
the primary transfer roller 53 by the primary transfer bias power source
54. A magnetic field is then formed in between the primary transfer
roller 53 and the photosensitive drum 1 in a direction which makes the
toner of the first polarity move from the photosensitive drum 1 to the
intermediate transfer belt 51. Thereby, the toner image on the
photosensitive drum 1 is transferred (primary transfer) to the surface of
the intermediate transfer belt 51 (primary transfer step). The process
speed corresponding to the surface movement speed of the photosensitive
drum 1 and the intermediate transfer belt 51 is 200 mm/sec.

[0032]Deposits such as toner (primary transfer residual toner) remaining
on the surface of the photosensitive drum 1 after the primary transfer
step are cleaned by the drum cleaner 6. The drum cleaner 6 has a cleaning
blade 61 as a drum cleaning member, a conveying screw 62, and a drum
cleaner housing 63. The cleaning blade 61 is abutted to the
photosensitive drum 1 at a prescribed angle and pressure by a pressure
unit (not illustrated). Thereby, toner remaining on the surface of the
photosensitive drum 1 is scratched off and removed from the
photosensitive drum 1 by the cleaning blade 61, and then collected within
the drum cleaner housing 63. Toner which has been collected is conveyed
by the conveying screw 62 and discharged to a discharge toner receptacle
(not illustrated).

[0033]Returning to FIG. 1, the constitution of the inside of the apparatus
main body 100A of the image forming apparatus 100 will be described
below. As illustrated in FIG. 1, below the photosensitive drums 1a to 1d,
the intermediate transfer belt 51, the primary transfer rollers 53a to
53d, the secondary transfer inner roller 56, the secondary transfer outer
roller 57, the intermediate transfer belt cleaner 59, and the like are
arranged to constitute an intermediate transfer unit 5. The secondary
transfer inner roller is electrically connected. Further, a secondary
transfer bias power source 58 is connected as a secondary transfer bias
output unit to the secondary transfer outer roller 57. The secondary
transfer inner roller 56 contacts the inner circumferential surface of
the intermediate transfer belt 51 and rotates in accordance with the
movement of the intermediate transfer belt 51.

[0034]For example, during formation of a full color image, a toner image
of each color is formed on the photosensitive drums 1a to 1d of the first
to fourth image forming portions Sa to Sd. The toner images of each color
receive a primary transfer bias from the primary transfer rollers 53
which are opposite to the photosensitive drums 1a to 1d to sandwich the
intermediate transfer belt 51, and then are transferred (primary
transfer) successively onto the intermediate transfer belt 51. These
toner images are conveyed to the secondary transfer portion N2 in
accordance with the rotation of the intermediate transfer belt 51.

[0035]Meanwhile, by this time, the recording material P is conveyed to the
secondary transfer portion N2 by a recording material supply unit 8.
Basically, in the recording material supply unit 8, the recording
material P, which is removed one sheet at a time by a pick up roller 82
from a cassette 81, which is a recording material container, is conveyed
to the secondary transfer portion N2 by a conveying roller 83.

[0036]During image formation, a secondary transfer bias voltage, which is
of an opposite polarity (second polarity: positive polarity in the first
embodiment) relative to the normal charging polarity (first polarity:
negative polarity in the first embodiment) of the toner, is applied to
the secondary transfer outer roller 57 by the secondary transfer bias
power source 58. A magnetic field is then formed in between the secondary
transfer inner roller 56 and the secondary transfer outer roller 57 in a
direction which makes the toner of the first polarity move from the
intermediate transfer belt 51 to the recording material P. Thereby, the
toner images on the intermediate transfer belt 51 are transferred
(secondary transfer) to the recording material P. The recording material
P, onto which the toner images have been transferred at the secondary
transfer portion N2 is conveyed to the fixing apparatus 7, which is a
fixing unit.

[0037]Deposits such as toner (secondary transfer residual toner) remaining
on the outer circumferential surface of the intermediate transfer belt 51
after the secondary transfer step are removed and collected by the
intermediate transfer belt cleaner 59. The intermediate transfer belt
cleaner 59 has the same constitution as the drum cleaner 6.

[0038]The fixing apparatus 7 has a fixing roller 71 which is rotatably
mounted and a pressure roller 72 which rotates while pressed to the
fixing roller 71. A heater 73 such as a halogen lamp is arranged within
the fixing roller 71. Temperature adjustment of the surface of the fixing
roller 71 is carried out by controlling the voltage supplied to the
heater 73. When the recording material P is conveyed to the fixing
apparatus 7, while the recording material P passes between the fixing
roller 71 and the pressure roller 72 which rotate at a fixed speed, the
recording material P is compressed and heated at an approximately fixed
pressure and fixed temperature from both the top and bottom surfaces.
Thereby, the unfixed toner images on the surface of the recording
material P are fused and fixed to the recording material P. In this way,
a full color image is formed on the recording material P.

[0039]The intermediate transfer belt 51 can include a dielectric resin
such as PC (polycarbonate), PET (polyethylene terephthalate), and PVDF
(polyvinylidene fluoride). As the intermediate transfer belt 51, a belt
formed by a PI (polyimide) resin having a thickness of 100 μm and a
surface resistivity of 1012Ω/quadrature (using a JIS-K6911
compliant probe, an applied voltage of 100 V, an applied time of 60 sec,
23° C./50% RH) is used. However, the belt is not limited to this
constitution, and a belt of another material having another volume
resistivity and thickness may be used.

[0040]As illustrated in FIG. 2, the primary transfer roller 53 includes
the metal core 531 having an outer diameter of 8 mm, and the conductive
layer 532 formed by a conductive urethane sponge having a thickness of 4
mm. The electrical resistivity of the primary transfer roller 53 was
approximately 105Ω (23° C./50% RH). The electrical
resistivity of the primary transfer roller 53 is calculated from the
current value measured when the primary transfer roller 53, which is
abutted by a metal roller grounded under a load of 500 g, is rotated at a
circumferential speed of 50 mm/sec and a 100 V voltage is applied to the
metal core 531.

[0041]As illustrated in FIG. 1, the secondary transfer inner roller 56
includes a metal core 561 having an outer diameter of 18 mm, and a
conductive, solid silicone rubber layer 562 having a thickness of 2 mm.
The electrical resistivity of the secondary transfer inner roller 56 was
approximately 104Ω as measured by the same measurement method
as the primary transfer roller 53.

[0042]As illustrated in FIG. 1, the secondary transfer outer roller 57
includes a metal core 571 having an outer diameter of 20 mm, and a
conductive EPDM rubber sponge layer 572 having a thickness of 4 mm. The
electrical resistivity of the secondary transfer outer roller 57 was
approximately 108Ω as measured by the same measurement method
as the primary transfer roller 53 when the voltage applied was 2000 V.

[0043]FIG. 3A is a cross-section view illustrating the constitution of the
optical sensor 30. As illustrated in FIG. 3A, the optical sensor 30,
which is a "first detection unit", is a sensor which faces a transfer
surface 51a of the intermediate transfer belt and is capable of detecting
the density of toner images transferred onto the transfer surface 51a.
The optical sensor 30 faces the transfer surface 51a of the intermediate
transfer belt 51, and one optical sensor 30 is arranged between the
primary transfer portion N1d for black, which is the fourth and last
color, and the secondary transfer portion N2 (refer to FIG. 1) and
exhibits a function for detecting a patch. The optical sensor 30 can
detect the density of a patch image by black toner formed on the surface
of the photosensitive drum 1d, as well as that of a patch image by
yellow-, magenta-, and cyan-colored toner formed on the surfaces of the
photosensitive drums 1a, 1b, 1c. The optical sensor 30 can also detect
the density of the ground of the intermediate transfer belt 51.

[0044]The optical sensor 30 includes a light-emitting element 30a and a
light-receiving element 30b. The light-emitting element 30a irradiates
light on the patch image of the transfer surface 51a of the intermediate
transfer belt 51. The light is regularly reflected or diffusedly
reflected at parts of the patch image on the surface of the intermediate
transfer belt 51, or it is regularly and diffusedly reflected, and then
proceeds to the light-receiving element 30b in a prescribed reflected
light quantity. The light-receiving element 30b receives the reflected
light quantity and generates an electrical signal (image density signal).
The light-receiving element 30b is connected to a density storage portion
50a of a controller 50.

[0045]The controller 50 illustrated in FIG. 1 will be described below. The
controller 50 includes a density storage portion 50a, a signal processing
portion 50d, a position calculation portion 50b, and a condition changing
portion 50c. The density storage portion 50a, which is a "storage unit",
is a part for storing detection results of the optical sensor 30 as
density information, which is a "profile" covering one round of the
intermediate transfer belt 51. The controller 50 amplifies an electrical
signal to a prescribed reference value with the signal processing portion
50b within the controller 50, and then stores this in the density storage
portion 50a as an amplification factor. The controller 50 provides
feedback for image forming conditions based on the obtained electrical
signal, and controls the image density by controlling toner replenishment
to developer and controlling conditions such as light exposure of a
latent image and developing bias. During installation of the apparatus
main body 100A or exchange of the intermediate transfer belt 51, first, a
profile of reflected light quantity (brightness) of the intermediate
transfer belt 51 is initially set.

[0046]A technology called ground correction is necessary for the
measurement of the patch density transferred to the transfer surface 51a
of the intermediate transfer belt 51 by the optical sensor 30. During
measurement of the patch density, ground correction has an effect of
canceling the influence from the gloss of the surface of the ground on
the patch density, while taking the condition of the ground of the
intermediate transfer belt 51 to which the toner is transferred into
account. The condition of the ground of the intermediate transfer belt 51
includes, for example, variation over time in the gloss of the transfer
surface 51a of the intermediate transfer belt and gloss irregularities in
the intermediate transfer belt 51 within one round.

[0047]In a state in which the output value of a sensor in the ground
directly below the toner patch of the intermediate transfer belt 51 has
been ascertained, ground correction (density correction) is carried out
by the method described below, for example. When a regularly reflected
light output R10 (toner patch) exists during toner patch detection and a
regularly reflected light output R20 (ground directly under toner patch)
exists during detection of the ground directly under the toner patch, the
toner patch density DENS (toner patch) is calculated by the following
formula (1):

[0048]FIG. 3B is a perspective view illustrating the constitution of a
reference position detection sensor 31. As illustrated in FIG. 3B, a
plurality of blank seals 32 are arranged on the edge of an opposite
surface 51b on the opposite side of the transfer surface 51a of the
intermediate transfer belt 51 in a direction orthogonal to the direction
of arrow K1, which is the direction of movement of the intermediate
transfer belt 51. The blank seals 32, which are a plurality of "position
marks", show the circumferential direction position, which is the
position in the circumferential direction of the intermediate transfer
belt 51. The blank seals 32 are utilized in order to obtain an output of
the optical sensor 30 for one round of the intermediate transfer belt 51.
The shape of each of the blank seals 32 will be described in detail
below, but basically the shapes are different at each circumferential
direction position, and thus the circumferential direction position of
the intermediate transfer belt 51 can be identified.

[0049]A reference position detection sensor 31, which is a "second
detection unit" capable of detecting the blank seals 32, is arranged
facing the intermediate transfer belt 51. The reference position
detection sensor 31 exhibits a function as a sensor which detects the
reference position of the intermediate transfer belt 51. The reference
position detection sensor 31 is connected to the position calculation
portion 50b (refer to FIG. 1) of the controller 50.

[0050]The above-described controller 50 will be described in further
detail below. The position calculation portion 50b, which is a
"calculation unit", is a part which calculates the circumferential
direction position of the intermediate transfer belt 51 based on a
detection result of the reference position detection sensor 31. The
controller 50 calculates the reference position of the intermediate
transfer belt 51 by detecting the reflected quantity of the blank seals
with the reference position detection sensor 31. Until the next blank
seal 32 comes, the position calculation portion 50b for calculating the
reference position in the controller 50 calculates the position in the
circumferential direction of the intermediate transfer belt 51 from the
rotation time and speed of the intermediate transfer belt 51. In other
words, the controller 50 determines the current phase of the intermediate
transfer belt 51 by the elapsed time from after the reference position
detection sensor 31 detects the reference position of the opposite
surface 51b of the intermediate transfer belt 51 and detects the position
in the circumferential direction of the intermediate transfer belt 51.

[0051]The above-described condition changing portion 50c, which is a
"changing unit", is a part which changes the conditions of the toner
images formed on the plurality of photosensitive drums 1a to 1d based on
density information, which is a "profile" stored in the density storage
portion 50a, and the circumferential direction position calculated by the
position calculation portion 50b. The controller 50 performs ground
correction of the intermediate transfer belt 51 based on the density of
the toner patch corresponding to the obtained phase of the intermediate
transfer belt 51.

[0052]First, in a state in which there are no deposits of toner on the
intermediate transfer belt 51, the reference position detection sensor 31
detects the blank seals 32. Subsequently, the optical sensor 30, as an
"optical density detection unit" which is a "first detection unit",
measures the reflected light quantity of the intermediate transfer belt
51 over one round of the intermediate transfer belt 51, and then stores
the output profile at that time in the density storage portion 50a
(control memory).

[0053]Next, initial setting of the developer density is carried out. The
initial setting of the developer density is also normally carried out
during installation of the apparatus main body 100A or exchange of the
developer. A latent image of the patch image, which is a reference image
for image density control, is formed on the photosensitive drum 1, and
then a developing bias is applied to the developing sleeve 42 of the
developing apparatus 4 to develop a patch latent image, and thereby a
patch image after primary transfer is obtained. After the reflected light
quantity from the patch image is measured by the optical sensor 30, the
reflected light quantity of the intermediate transfer belt 51 at the
patch image formation position is calculated from the output from the
above-described reference position detection sensor 31 and the stored
reflected light quantity profile of the intermediate transfer belt 51.
Next, the patch image density is calculated from Formula (1). The image
density signal is read as an initial setting value, and then stored in a
control memory (not illustrated).

[0054]Next, after use in image formation, at an appropriate time, for
example, after every certain number of sheets of image formation, a patch
image is formed, the reflected light quantity of the patch image is
measured with the optical sensor 30, and the patch image density is
calculated from Formula (1) using the reflected light quantity of the
patch image formation position. The obtained image density signal is then
compared with an initial value, the comparison result is fed back to the
image formation conditions, and then the image density is controlled by
controlling toner replenishment to developer and controlling conditions
such as light exposure of a latent image and developing bias. Here, a
regular reflection-type of sensor was used as the optical sensor 30, but
the optical sensor 30 is not limited to a regular reflection-type.

[0055]FIG. 4A is a schematic view illustrating the shapes of blank seals
32 extending in the circumferential direction of the intermediate
transfer belt 51. As illustrated in FIG. 4A, for the blank seals 32 as
"belt position detecting marks", which are "position marks", seals having
different lengths in the circumferential direction of the intermediate
transfer belt 51 are used. In this way, since the dimensions in the
circumferential direction of the intermediate transfer belt 51 are
different, the shapes of each of the blank seals 32 are different. For
example, four types of the blank seals 32 can be used. When expressed in
terms of the length in the width direction of the intermediate transfer
belt 51×the length in the circumferential direction of the
intermediate transfer belt 51, the dimensions of the blank seals 32 are 1
cm×1 cm, 1 cm×1.5 cm, 1 cm×2 cm, and 1 cm×2.5 cm.
Here, the blank seals 32 are used as the "position marks", but the
"position marks" are not limited to the blanks seals 32. The "position
marks" are provided for ascertaining the reference position of the
intermediate transfer belt 51, and although they are not used here for
the timing of image formation, they can be used for the timing of image
formation. As discussed above with reference to FIG. 3B, an optical
sensor is used as the reference position detection sensor 31, which
functions as the "belt position mark detection sensor", which is the
"second detection unit". Since the length in the circumferential
direction of the intermediate transfer belt 51 of each of the blank seals
32 is different, when the reference position detection sensor 31
irradiates light on each of the four blank seals 32 described above, the
reflection time from the blank seals 32 changes. The reference position
detection sensor 31 can distinguish between each of the blank seals 32 by
the difference in the reflection time.

[0056]FIG. 4B is a graph illustrating the relationship of the pattern
output from the reference position detection sensor 31 and the time. As
illustrated in FIG. 4B, the output of the reference position detection
sensor 31 corresponds to the dimensions of the blank seals 32 in the
width direction of the intermediate transfer belt 51, and the time of
output corresponds to the dimensions of the blank seals 32 in the
circumferential direction of the intermediate transfer belt 51.

[0057]FIG. 4C is a graph illustrating the relationship of the belt ground
output of the intermediate transfer belt 51 and the time. Upon entering a
sequence in which the profile of the light reflectance of the ground of
the intermediate transfer belt 51 is acquired, the controller 50 receives
and stores detection information of the blank seals 32 detected by the
reference position detection sensor 31, and then begins to acquire the
profile of one round of the intermediate transfer belt 51. As illustrated
in FIG. 4C, each time the controller 50 detects a blank seal 32 having a
different shape, the position of the profile corresponding to the blank
seal 32 is stored. The acquisition of the profile is complete once a
blank seal 32 having the same shape is obtained again.

[0058]By this sequence, regardless of when the patch image was transferred
to the intermediate transfer belt 51, the controller 50 can calculate the
position of the intermediate transfer belt 51 from the elapsed time after
a blank seal 32 has passed and the rotation speed of the intermediate
transfer belt 51. As a result, the controller 50 can derive a profile
corresponding to the position of the intermediate transfer belt 51 at
each position in the circumferential direction of the intermediate
transfer belt 51.

[0059]For example, when a patch image is actually transferred onto the
intermediate transfer belt 51, the controller 50 operates as described
below in order to retrieve a profile of the ground directly below the
patch image. The controller 50 calculates the absolute position on the
intermediate transfer belt 51 from the timing at which the patch image
was imaged and the elapsed time from the time at which the reference
position detection sensor 31 lastly detected a blank seal 32. The
controller 50 then finds the belt ground reflection efficiency in the
corresponding profile from FIG. 4C based on the absolute position.
Thereby, the controller 50 can calculate the actual density of the patch
image from the above-described Formula (1). In the first embodiment, the
light reflectance profile of the belt ground is used as the profile for
one round of the intermediate transfer belt 51, but the present invention
is not limited to this constitution.

[0060][Timing of the Profile Acquisition Sequence For One Round of the
Intermediate Transfer Belt 51]

[0061]Regarding the timing for measuring the light reflection profile of
the intermediate transfer belt 51, this measurement is carried out during
installation of the apparatus main body 100A, during exchange of the
intermediate transfer belt 51, and at intervals sufficiently longer than
the patch image formation frequency. For example, if the patch image
formation frequency is set at intervals of every 100 sheets of image
formation, the frequency of measuring the profile of one round of the
intermediate transfer belt 51 is set at intervals of every 10,000 sheets
of image formation. However, the measurement timing is not limited to
this timing.

[0062]In the first embodiment, the reflectance of the belt ground was used
as the profile of one round of the intermediate transfer belt 51.
However, the profile is not limited to the reflectance, and the thickness
profile of the intermediate transfer belt 51 can be detected and this can
be fed back to the belt rotation speed.

Second Embodiment

[0063]FIG. 5A is a schematic view illustrating the shapes of blank seals
232 of the intermediate transfer belt 51 according to a second
embodiment. In the explanation below about the image forming apparatus of
the second embodiment, constitutions and effects of the image forming
apparatus of the second embodiment which are identical to those of the
image forming apparatus 100 of the first embodiment will be assigned
identical reference numerals, and descriptions thereof will be
appropriately omitted. The image forming apparatus of the second
embodiment differs from the image forming apparatus 100 of the first
embodiment in that the shapes of each of the blank seals 232a to 232c
differ from each other in their dimensions in the width direction of the
intermediate transfer belt 51. In more detail, in the image forming
apparatus of the second embodiment, the shape of the first blank seal
232a among the blank seals 232, which function as "belt position
detection marks", which are "position marks", is a square shape. The
shape of the second blank seal 232b is a triangle shape which is convex
toward the upstream side in the circumferential direction of the
intermediate transfer belt 51. The shape of the third blank seal 232c is
a triangle shape which is convex toward the downstream side in the
circumferential direction of the intermediate transfer belt 51. In this
way, the light reflectance of each of the blank seals 32 is different at
each position in the circumferential direction, and thus the
circumferential direction position of the intermediate transfer belt 51
can be identified. Thereby, in the image forming apparatus of the second
embodiment, the position of the intermediate transfer belt 51 can be
derived from the shapes of the blank seals 232a to 232c.

[0064]In further detail, three types of seals having different shapes are
used for the blank seals 232a to 232c. The blank seal 232a is a square in
which each side is 1 cm. The blank seal 232b is an equilateral triangle
in which each side is 1 cm formed to be convex toward the upstream side
in the circumferential direction of the intermediate transfer belt 51.
The blank seal 232c is an equilateral triangle in which each side is 1 cm
formed to be convex toward the downstream side in the circumferential
direction of the intermediate transfer belt 51. However, the blank seals
are not limited to these shapes. As discussed above with reference to
FIG. 3B, an optical sensor is used as the reference position detection
sensor 31, which functions as a "belt position mark detection sensor",
which is the "second detection unit". Since the shape of each of the
blank seals 232a to 232c is different, when the reference position
detection sensor 31 irradiates light on each of the three blank seals
232a to 232c described above, the reflection intensity from the blank
seals 232a to 232c changes. The reference position detection sensor 31
can distinguish between each of the blank seals 232a to 232c by the
difference in the reflection intensity.

[0065]FIG. 5B is a graph illustrating the relationship of the pattern
output from the reference position detection sensor 31 and the time. As
illustrated in FIG. 5B, the output of the reference position detection
sensor 31 corresponds to the dimensions of the blank seals 32 in the
width direction of the intermediate transfer belt 51, and the time of
output corresponds to the dimensions of the blank seals 32 in the
circumferential direction of the intermediate transfer belt 51.

Third Embodiment

[0066]FIG. 6A is a schematic view illustrating the shapes of blank seals
332 of the intermediate transfer belt 51 according to a third embodiment.
In the explanation below about the image forming apparatus of the third
embodiment, constitutions and effects of the image forming apparatus of
the third embodiment which are identical to those of the image forming
apparatus 100 of the first embodiment will be assigned identical
reference numerals, and descriptions thereof will be appropriately
omitted. The image forming apparatus of the third embodiment differs from
the image forming apparatus 100 of the first embodiment in that the
number of unit figures K extending in the width direction of the
intermediate transfer belt 51 which constitute each of the blank seals
332a to 332c differs in each of the blank seals 332a to 332c. In more
detail, the shape of the first blank seal 332a among the blank seals 332,
which function as "belt position detection marks", which are "position
marks", is a first pattern including one rectangular unit figure K
extending in a strip shape in the axial direction of the photosensitive
drums 1. The shape of the second blank seal 332b is a second pattern
including two rectangular unit figures K extending in a strip shape in
the width direction of the intermediate transfer belt 51. The shape of
the third blank seal 332c is a third pattern including three rectangular
unit figures K extending in the width direction of the intermediate
transfer belt 51. Among these three types of blank seals 332a to 332c,
the number of unit figures K constituting the blank seals 332a to 332c is
different at each circumferential direction position, and thus the
circumferential direction position of the intermediate transfer belt 51
can be identified. Thereby, in the image forming apparatus of the third
embodiment, the position of the intermediate transfer belt 51 can be
derived from the shapes of 332a to 332c of the unit figures K.

[0067]Three types of seals having different shapes are used for the blank
seals 332a to 332c. The blank seals 332a to 332c are different from those
in the first and second embodiments described above. Basically, first,
the shape of the unit figures K themselves are a dimension of 1 cm in the
axial direction of the photosensitive drums 1 and a dimension of 0.5 cm
in the circumferential direction of the intermediate transfer belt 51. In
a first position, one unit figure K is adhered. In a second position, two
unit figures K are adhered with a space of 0.5 cm in between them. In a
third position, three unit figures K are adhered with a space of 0.5 cm
in between them. However, the shapes or patterns of the blank seals are
not limited to those above. As discussed above with reference to FIG. 3B,
an optical sensor is used as the reference position detection sensor 31,
which functions as a "belt position mark detection sensor", which is the
"second detection unit". Since the shape of each of the blank seals 332a
to 332c is different, when the reference position detection sensor
irradiates light on each of the three blank seals 332a to 332c described
above, the number of reflections from the blank seals 332a to 332c
changes. The reference position detection sensor 31 can distinguish
between each of the blank seals 32 by the difference in the number of
reflections.

[0068]FIG. 6B is a graph illustrating the relationship of the pattern
output from the reference position detection sensor 31 and the time. As
illustrated in FIG. 5B, the output of the reference position detection
sensor 31 corresponds to the dimensions of the blank seals 332 in the
axial direction of the photosensitive drums 1, and the time of output
corresponds to the dimensions of the blank seals 332 in the
circumferential direction of the intermediate transfer belt 51.

[0069]According to the first to third embodiments, a plurality of blank
seals are different from each other at each circumferential direction
position. Therefore, even if the position calculation portion 50b loses
the information of the circumferential direction position, it can
recognize the closest circumferential direction position existing on the
upstream side in the rotation direction of the intermediate transfer belt
51 to determine the position in the circumferential direction of the
intermediate transfer belt 51. As a result, the time required for the
controller to determine the position of the intermediate transfer belt 51
is reduced. Therefore, the reference position of the intermediate
transfer belt 51 can be quickly detected, and the occurrence of a wait
time can be eliminated by accurately distinguishing between the plurality
of blank seals.

[0070]According to the first and second embodiments, the shapes or light
reflectance of each of a plurality of blank seals is different, and thus
the ability to identify the circumferential direction position of the
intermediate transfer belt 51 is enhanced.

[0071]According to the third embodiment, the number of unit figures K
constituting the blank seals 332a to 332c is different for each of the
blank seals 332a to 332c at each position in the circumferential
direction. Since the shapes of the seals are different, the ability to
identify the circumferential direction position of the intermediate
transfer belt 51 is enhanced.

[0072]In the first to third embodiments described above, one among the
shape, light reflectance, and number of unit figures K constituting the
blank seals is different for each of the blank seals 32, 232, 332 at each
position in the circumferential direction, and thus the circumferential
direction position of the intermediate transfer belt 51 can be
identified. However, the present invention is not limited to these
embodiments. Members having a magnet can also be used for the blank seals
32, 232, 332, and the plurality of blank seals 32, 232, 332 can be
identified by the magnet.

[0073]In the first to third embodiments described above, an intermediate
transfer system was described. However, the constitution of the present
invention can also be applied to a mechanism utilizing a direct transfer
system.

[0074]According to the present invention, a plurality of position marks
differ from each other at each position in the circumferential direction.
Therefore, even if the calculation unit loses the information of the
circumferential direction position, it can recognize the closest
circumferential direction position existing on the upstream side in the
rotation direction of the belt member to determine the position in the
circumferential direction of the belt member. As a result, the time
required for the calculation unit to determine the position of the belt
member is reduced.

[0075]Embodiments of the present invention were described above, but the
present invention is not limited to the above embodiments, and any kind
of modification can be made within the technical concept of the present
invention.

[0076]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0077]This application claims the benefit of Japanese Patent Application
No. 2009-179899, filed Jul. 31, 2009, which is hereby incorporated by
reference herein in its entirety.

Patent applications by CANON KABUSHIKI KAISHA

Patent applications in class Having detection of toner (e.g., patch)

Patent applications in all subclasses Having detection of toner (e.g., patch)